Rectal Live-Animal Scrapie Test Approved By USDA

By Sarah Watson, Livestock Editor
Thursday, May 22, 2008 1:08 PM CDT

Earlier this year the USDA’s Animal and Plant Health Inspection Service approved a new live-animal scrapie test for both sheep and goats.

Similar to the currently used third eyelid test, the test involves collecting lymphoid tissue. The new test, however, uses rectal mucosa biopsy, as opposed to third eyelid biopsy. Both tests can be conducted on live animals using local anesthetic.

The rectal mucosa biopsy test is easier for both the sample collector - a vet or animal health technician - and the animal involved.

A speculum tool is used to gain access into the rectal area, local anesthetic is applied and the sample is collected

“You just snip off a very small piece of tissue about the size of a penny to a nickel,” says Dr. Diane Sutton, National Scrapie Program coordinator. “You’re just taking off the very superficial layer. You’re not cutting down into the muscle.”

The test is easier to conduct because the eye is more difficult to take a sample from and the animal is easier to handle.

“The animal would much rather have you poking around in that end than poking around near their eye,” Sutton explains. “So there’s a lot less (stress) on the animal due to restraint involved.”

The collection is also better scientifically.

“There’s a lower no-test rate,” she said, comparing the two live-tests available. There is a greater number of lymphoid follicles in the rectum than in the eyelid.

As technicians continue to conduct the rectal test the better they will become and reduce the number of no-tests. Currently the no-test rate for the third eyelid is close to 30 to 40 percent and Sutton believes the rectal sample no-test could get down around 10 percent.

“I think it will get lower as technicians gain experience,” she says.

In 2007, VS conducted a large-scale field study to evaluate rectal biopsy as a means to collect lymphoid tissue for scrapie testing. Using live, high-risk sheep and goats, the study compared the test results from rectal biopsies and third eyelid biopsies to test results obtained postmortem from the same animals on brainstem, lymph node, tonsil and rectal biopsy.

The study found that testing live-animal rectal biopsy samples is an effective means of detecting scrapie. The rectal biopsy testing detected 87 percent of the scrapie positive animals that were determined to be positive by testing tissues collected postmortem.

Live-animal, rectal biopsy testing has a low rate of complications and is faster and easier than other live-animal testing methods. The study also showed that there was strong agreement in test results from multiple rectal biopsy sites in the same animal. This will allow for repeat testing over time if needed.

However not as good as a test on necropsy, where both the lymph nodes and brain are available for testing and ample tissue will be available for samples. However, the necropsy test requires the animal be dead, she explained.

The current goal of the National Scrapie eradication program was to be “substantially eradicated” by 2010.

“I think we are going to come fairly close to that,” she said. “It might take until 2013. It will take us several years beyond that before we see the absolutely last case.

Scrapie eradication is a huge road block for trade.

“The primary reason for eradicating it is it’s a trade issue,” she explained. “As long as we have it we can’t export live sheep and goats to most other countries.”

Scrapie is a fatal, degenerative disease affecting the central nervous system of sheep and goats.

Infected flocks that contain a high percentage of susceptible animals can experience significant production losses. Prior to approval of third eyelid biopsy, definitive diagnosis of scrapie only could be made following necropsy and examination of brain, lymph node or tonsil or by general anesthesia and collection of lymph node or tonsil.

Live-animal testing using local anesthesia provides a practical method of detecting scrapie-infected animals before development of clinical signs, which could assist producers in eradicating the disease from their flocks.

In the future the rectal test will be used significantly more than the third eyelid test, Sutton says.

“I think you’ll see an increase in live animal surveillance activity being done,” she said.

The possibility to determine scrapie before death will help producers control and monitor the disease and possibly track the disease.

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